The distribution of the list to other libraries was not, at first, contemplated. The printing of the subject headings was undertaken to facilitate the work of the catalogers in the Library of Congress, and it was believed that, if supplied to other libraries in its preliminary form, the list would give rise to many queries in regard to unavoidable omissions and inconsistencies. It had not progressed far, however, before many libraries intimated that it would be useful to them to receive the letters as they were issued, and when requests became too urgent for refusal, it was decided to supply copies at a price insuring that only those libraries should order them which had serious use for them. It was considered that 50 copies for distribution outside the Library of Congress would surely be sufficient, but it turned out that the estimate was too small, and, in consequence, there has had to be much reprinting of the early letters of the alphabet. The edition of the letter P, just issued, was 500 copies.
The scope of the list of headings is largely inclusive in its character, covering subjects in all branches of knowledge as far as they have been adopted in the Library of Congress catalog. The names of persons and places are, however, omitted, also names of societies, institutions, and bodies of various kinds, names of treaties and conventions, and systematic names of genera and species in botany and zoology.
The classes theology, and military and naval science are only partially represented in the list, as these sections are not yet re-cataloged. The classes language, literature, and philology, which are now in the process of recataloging, are more fully, but not yet wholly, represented. In the earlier letters of the alphabet, few headings in law were introduced (as it has only been during the past few months that the law headings have been systematically considered), but they are now included in the list, and many of those omitted in the earlier letters are being entered in the lists of additions to the subject headings issued in connection with the main list.
We include in the list the more important subdivisions under a subject. These subdivisions are printed in italics, and separated from the main subject by a dash. One point to which I would especially call the attention of librarians using the list is that ordinarily only those subdivisions are printed under a subject which are distinctive, or peculiar to that subject. General form subdivisions, such as Directories, Periodicals, Societies, etc., which may properly be used under any subject requiring them, are, as a rule, omitted from the list. (A list of these form subdivisions can be found on p. 19 of the "Preliminary list of subject subdivisions," issued by the library in 1910.) Under names of countries only the history subdivisions are included.
Turning now from the consideration of the subdivisions, a few words may be useful in regard to the cross-references from subject headings to related subjects. In general, it may be said that references are made from the more inclusive to the smaller subjects, and not ordinarily back from smaller to larger. We should refer from Grain to Maize and Rye, but not from Maize and Rye back again to Grain. Where practicable, references are made from the most inclusive to somewhat more limited subjects, and from these latter to subjects still more specific, rather than from the inclusive to the specific subjects. We refer from Art to Engraving, from Engraving to Stipple-engraving, not directly from Art to Stipple-engraving. These general principles have been departed from where it has seemed expedient, the desire being to render the list useful and practical, rather than to make it adhere too strictly to rigid rules of procedure.
The seeming incompleteness of references from many subjects, references which obviously are needed to round out the various aspects of subjects is due to the fact that certain headings are not as yet introduced in the Library of Congress catalog. We have been very conservative about introducing new headings until called for by the books in hand, judging that the headings should be made to conform to the literature, rather than the literature to the headings.
The printing of a subject in antique type indicates that, in the library catalog, the subject has country subdivision, as in Education, Labor and laboring classes, Insurance, etc. It may be helpful to add here that the country is subordinated to the subject in our catalog, when it seems desirable to keep the material on a topic together, rather than to distribute it under the country headings. This includes many subjects in technology, science, art, and the social sciences.
The numbers which follow the subject headings indicate where the material dealing with those subjects is classified in the Library of Congress. The explanatory words following these numbers serve merely to guide those interested in the classification scheme. They are in different form from the subject headings, and should not be confused with them. In the matter of hyphens, the Century dictionary has been used as an authority.
At the present time the list of headings has been completed through the letter P. Q and R are now ready for the press, and will probably be issued in the course of a few weeks. The editor of the list sometimes feels it to be a cause for gratitude that the English alphabet is composed of only 26 letters. Should it contain as many letters as some other alphabets, the Sanskrit for example, the day of completion of the list might indeed be far away.
A few words in regard to the printing of the lists known as "Additions and corrections" will, I think, be needed for a full understanding of the subject headings. I have already spoken of the lists of additions issued in connection with the old A. L. A. list, before the Library of Congress list of headings began to be printed. When letter A of our new list was ready for press, there had been four of these lists issued, the additions being cumulated in each successive number. The corrections in the lists appeared but once, and were carried over by the catalogers to copies of the A. L. A. list. The headings in these early supplementary lists have, of course, been incorporated in the Library of Congress list, as far as the letters have been printed.